No electoral method is perfect and there are problems with Instant runoff ranked ballots. Look up "Favourite Betrayal" as a problem where ranking your most favourite incorrectly can accidentally result in your least favourite winning.

Of course just because there are technically flaws, doesn't mean it's not an improvement over the status quo.

Instant Runoff Ranked ballots are particularly well suited for electing a single position like a Mayor or President. Given that it's still a majoritarian system like First Pass the Post, it's a much more questionable system to use for other things like electing a parliament.

Depends on which ranked system. IRV has a lot of issues that other systems (like STAR, my personal preference in voting system, or Condorcet methods for instance) address much better. Non-monotonicity in particular seems like a bad property of a voting system, so I'm confused that IRV is so popular recently.

_Is_ it lacking? Most democratic systems use some form of PR and/or runoffs.

First past the post is beneficial for establishment parties.